This invention relates to pole changeable three phase windings in general and more particularly to a pole changeable three phase winding for a fractional pole pair ratio of the first and second numbers of pole pairs p.sub.1 and p.sub.2 according to p.sub.1 :p.sub.2 =(3m.+-.1):3n, where p.sub.2 =3.sub.n is an integral multiple of the number of phases and m and n are positive integers.
A winding of the above type is disclosed in my U.S. Pat. No. 4,144,470 which issued on Mar. 13, 1979, the disclosure of which is incorporated herein by reference.
According to my '470 patent, each phase winding for the first number of poles 2p.sub.1 consists of 2p.sub.1 /t equal winding branches, of which a number G, divisible by three, of so-called base winding-branches carry current for the first number of poles 2p.sub.1 and for the second number of poles 2p.sub.2, where one third of each of the base winding branches, for the second number of poles 2p.sub.2, are assigned to all three phase windings. The remaining number N of winding branches are designed as zero or null winding branches which are ineffective for the second number of poles 2p.sub.2, so that a division into G base winding branches and N zero branches according to G+N=2p.sub.1 takes takes place, where t is an integral divisor of 2p.sub.1.
The G base-winding branches may be connected in triple-Y/triple Y; delta/triple-delta; or Y/triple-delta. In a case of triple-Y/triple Y connection of the G base-winding branches, only six terminals are needed, and for the selective insertion of one of the two pole numbers, a single three-pole switch is sufficient. The null winding can be connected as desired, parallel to or in series with the base winding; however, only the use of a single null branch winding for the primary pole pair number p.sub.1 not divisible by three is provided.
To attain favorable operating properties, in particular for drives at constant torque, it is desirable to obtain for both pole number levels equal air gap flux densities B to the extent possible, the ratio of the two flux densities to be not less than 0.85, if possible. In a winding with base-winding branches connected in triple-Y/triple-Y and parallel null winding branches, the phase turn numbers for both pole pair number levels are the same, so that for p.sub.2 &gt;p.sub.1 -because of the limited magnitude of the higher-pole winding factor .xi..sub.2 -there is to be provided for the lower primary pole number a phase-interspersed winding arrangement maintaining the winding and field symmetry and having a correspondingly reduced winding factor. As a result, however, the copper losses increase approximately as the square of the reciprocal winding factor at the lower primary pole number, the phase turn numbers being the same for both pole number levels. The essential operating properties depend on the effective turn numbers, which result from the product of winding factor and actual turn number.